The present invention relates to a vacuum arc remelting ("VAR") apparatus and process.
VAR is a process for controlled solidification of segregation-sensitive alloys. In the process, a cylindrically shaped, alloy electrode is loaded into a water-cooled, copper crucible of a furnace. The furnace is evacuated and a dc electrical arc is struck between the electrode (cathode) and some start material at the bottom of the crucible (anode). The arc heats both the start material and the electrode tip, eventually melting both. As the electrode tip is melted away, molten metal drips into the crucible below. The process maintains a liquid melt pool that extends down to a mushy region, which is a transition zone to a fully solidified ingot. The crucible diameter is larger than the electrode diameter. Consequently, the ever-shrinking electrode can be translated downwards toward the anode pool surface to keep constant a mean distance between the electrode tip and the pool. The mean distance from the electrode tip to the liquid metal pool surface is called the electrode gap (g.sub.e).
As cooling water extracts heat from the crucible wall, molten metal next to the wall solidifies. A solid layer of material solidifying against the crucible wall near the pool surface is called a "shelf." At some distance below the molten pool surface, material becomes completely solidified, yielding a fully dense alloy ingot. After a sufficient period of time has elapsed, a steady-state situation evolves, consisting of a shelfed "bowl" of molten material situated on top of a fully solidified ingot base.
VAR converts material electrodes into ingots having refined grain size and improved chemical and physical homogeneity. VAR is particularly suited to melting nickel-based "superalloys" (such as Alloy 718). These materials contain substantial quantities of reactive elements. VAR reduces contained gases, especially hydrogen and oxygen, non-metallic inclusions and center porosity and segregation. Mechanical properties of the remelted alloy, such as ductility and fatigue strength, are improved.
During the VAR process, volatile contaminate species such as manganese, aluminum and chromium evaporate. The vapor species of these elements condense on cold surfaces such as the area of a crucible wall immediately above the shelf of freezing material. Additionally as the electrode arc moves about the surface of the electrode, some particles splatter out of the melt pool and against the crucible wall where they can be trapped by the forming skin of the condensing vapor species.
As the shelf forms, high-melting-point solute-lean material is the first liquid metal to freeze against the condensed volatile species and splatter that covers the crucible wall. Additionally, as a melt proceeds, oxide and nitride inclusions present on the surface of the liquid metal pool are commonly pushed off to the sides of the melt pool and are frozen into solidified material at the shelf.
As the electrode melts off and liquid metal fills the crucible, the ingot shelf melts from the underside while a new shelf forms on the upperside. If a steady state of melting and shelf forming is maintained, then the shelf progressively forms and melts and progresses upward with the surface of the melt pool. So long as the steady state persists, the shelf acts as a barrier between the freezing melt splatter and condensing vapor species against the crucible wall. However if the steady state cannot be maintained, the shelf becomes unstable, breaks off and falls into the melt pool, dragging along vapor species skin, splatter and high-melting-point solute-lean material. The solute-lean material will appear in the ingot as a shiny "white spot." If the solute-lean material is accompanied by oxide species then the solute-lean material appears as a "dirty white spot." These areas of solute-lean material and oxide species are sites for early failure initiation, resulting in reduced life of parts made from the material.
There is a need for a VAR furnace and process that avoid contamination of the melt with areas of splatter and oxide species.